ABSTRACT
We live among millions of microorganisms, whose ubiquitous communities have a profound impact on our health. A large variety of microbial populations (microbiota) and their genetic apparatus (microbiome) playing a substantial role in the maintenance of health and in the onset of disease. While the human genome is basically stable (Weinstock, 2011), the microbiome – the genetic material pertaining to all the microbes that live within the human body and, therefore, represents a sort of second human genome (according to Weinstock GM) – undergoes significant changes, varying not only between individuals but also within the same person. Viruses, bacteria, and eukaryotic microorganisms coexist in complex communities, and can interact with the micro-and macroenvironments in which they live, influencing the development, metabolism, and functions of higher organisms – namely us (Relman, 2011). At any one time, we can play host to up to three types of microorganisms. The first type, considered native or resident, comprises a population arising from the first colonization after the beginning of extra-uterine life. We can also be colonized by transient populations, which invade through a lumen without causing major changes to the resident bacterial population, as well as contaminant or frankly pathogenic microorganisms, which differ from the former by causing dismicrobism and the onset of infectious disease. Each such microbial population thrives in different “environmental” conditions (pH, temperature, etc.), and reacts differently to the action of drugs and/or exogenous chemicals (antimicrobials, enzymes, etc.). They also differ in terms of developmental changes to the genome, which affect the so-called quasispecies as a whole, and must compete for their microbial and nutritional needs, etc.
